Combustion control device for an engine

ABSTRACT

A sub-piston 19 is supported movably in the upward and downward direction on an upper end of a main piston 4 to define an air-fuel mixture cooling chamber 20 between the pistons 4 and 19. The air-fuel mixture cooling chamber 20 communicates with a peripheral edge of a combustion chamber 12. The sub-piston 19 is connected to a cam member 23 which is supported on a crankshaft 7 through a subsidiary connecting rod 21. The volume of the air-fuel mixture cooling chamber 20 is increased and decreased in operative association with the rotation of the crankshaft 7. The air-fuel mixture cooling chamber 20 has an increased volume in a phase from a compression stroke to a point immediately after ignition, and the generation of a knocking is prevented by cooling an air-fuel mixture filled in such air-fuel mixture cooling chamber 20. The volume of the air-fuel mixture cooling chamber 20 is decreased from the instant immediately after ignition, thereby causing the air-fuel mixture in the air-fuel mixture cooling chamber 20 to be pushed into the combustion chamber 12 and burned therein. Thus, even if the compression ratio is increased and the ignition timing is advanced to provide an enhancement in thermal efficiency, it can be ensured that a knocking is difficult to occur.

TECHNICAL FIELD

The present invention relates to a combustion control system for anengine in which a crankshaft is driven for rotation by burning anair-fuel mixture in a combustion chamber.

BACKGROUND ART

A broken line in FIG. 15 indicates a theoretical thermal efficiency inan Otto-cycle engine, and it is known that the thermal efficiency is afunction of only a compression ratio and is increased with an increasein the compression ratio. However, if the compression ratio isincreased, a knocking is liable to be generated. For this reason, theactual circumstances are that the engine is operated with a thermalefficiency lower than the theoretical thermal efficiency by retardingthe ignition timing to lower the maximum pressure and temperature. Theknocking is liable to be generated in a lower-speed rotational range andhence, the thermal efficiency is significantly reduced in thelower-speed rotational range as illustrated in FIG. 15. This is one offactors which impede the development of the engine in which a sufficienttorque is produced at a lower speed.

The knocking in the lower-speed rotational range means a phenomenon inwhich an unburned air-fuel mixture is adiabatically compressed into ahigher temperature and a higher pressure by the expansion of acombustion gas in an already burned area with progression of a surfaceof flame after ignition, and such unburned air-fuel mixture isself-fired before arrival of a normal surface of flame. The lower thenumber of rotations, the lower the burning speed, and the longer thetime expired up to the arrival of the normal surface of flame and hence,the knocking is liable to occur. In this case, if the knocking isavoided by retarding the ignition timing to lower the maximum pressureand temperature, as described above, a reduction in thermal efficiencyis brought about.

Therefore, there is a proposed technique for inhibiting the knockingwithout reducing the thermal efficiency, wherein the burning speed isincreased so that a normal combustion is completed in a time as short aspossible. In this case, for example, a squish area is provided to supplyan intense flow to the air-fuel mixture and to supply a swirl or atumble flow to intake air. There is also a proposed technique in whichin order to reduce the burning distance to complete the normal burningin a short time, a spark plug is disposed in a central portion of acombustion chamber, and the combustion chamber is formed into a shapenear a spherical shape.

However, if the speed of burning of the air-fuel mixture is increased inorder to inhibit the knocking, the following problem is encountered: Acombustion gas having a higher temperature is brought into contact witha wall surface of the combustion chamber and an upper surface of apiston at a higher speed and hence, the heat loss is increased due to anincrease in heat transfer rate to reduce the thermal efficiency as aresult. For this reason, an enhancement in thermal efficiencycorresponding to an increment in compression ratio cannot be obtained.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to reliablyinhibit the knocking in an internal combustion engine without reducingthe thermal efficiency.

To achieve the above object, according to a first aspect and feature ofthe present invention, there is provided a combustion control system foran engine, comprising an air-fuel mixture cooling chamber whichcommunicates with a combustion chamber defined in a cylinder head, sothat an air-fuel mixture can flow from the air-fuel mixture coolingchamber into the combustion chamber and vice versa, and acooling-chamber volume changing means for increasing and decreasing avolume of the air-fuel mixture cooling chamber in operative associationwith a rotation of a crankshaft, wherein the cooling-chamber volumechanging means is operable to decrease the volume of the air-fuelmixture cooling chamber during burning of the air-fuel mixture in thecombustion chamber.

With the first feature of the present invention, while the air-fuelmixture in the combustion chamber is fired and a surface of resultingflame is propagated, the air-fuel mixture in the air-fuel mixturecooling chamber is cooled into a reduced temperature. Therefore, theair-fuel mixture is difficult to self-fire, thereby inhibiting thegeneration of a knocking. Thus, it is possible to perform an increase incompression ratio and an advance of the ignition timing withoutgeneration of the knocking, thereby enhancing the thermal efficiency.Especially, in a lower-speed rotational range in which a knocking isliable to be generated, an increase in torque can be achieved. Moreover,in the course of propagation of the surface of flame produced from theair-fuel mixture fired in the combustion chamber, the air-fuel mixturepushed out of the air-fuel mixture cooling chamber having a decreasedvolume into the combustion chamber is burned with a time lag. Therefore,the burning of the air-fuel mixture can be slowly conducted to inhibitthe generation of NO_(x) and to alleviate the vibration and noise.

According to a second aspect and feature of the present invention, inaddition to the first feature, a sub-piston is supported movably inupward and downward direction on a main piston which is slidablyreceived in a cylinder bore, and the air-fuel mixture cooling chamber isdefined between an upper surface of the main piston and a lower surfaceof the sub-piston.

With the second feature of the present invention, the air-fuel mixturecooling chamber connected to the combustion chamber can be easilyformed.

According to a third aspect and feature of the present invention, inaddition to the second feature, a subsidiary connecting rod isrelatively movably accommodated in a main connecting rod which connectsthe main piston and the crankshaft to each other, the subsidiaryconnecting rod being connected at an upper end thereof to the sub-pistonand at a lower end thereof to a cam member which is rotatably supportedon the crankshaft.

With the third feature of the present invention, the sub-pistonsupported on the main piston which is movable within the cylinder borecan be moved up and down in operative association with the crankshaft.

According to a fourth aspect and feature of the present invention, inaddition to the third feature, the cam member is connected to a casingthrough a gear train, so that the cam member is rotated in operativeassociation with the rotation of the crankshaft.

With the fourth feature of the present invention, the cam member can berotated in precise operative association with the rotation of thecrankshaft.

According to a fifth aspect and feature of the present invention, inaddition to the fourth feature, the cam member performs one rotation forevery two rotations of the crankshaft, and the volume of the air-fuelmixture cooling chamber is increased and decreased one time for everytwo rotations of the crankshaft.

With the fifth feature of the present invention, the volume of theair-fuel mixture cooling chamber can be controlled to the timing ofignition of the 4-cycle engine.

According to a sixth aspect and feature of the present invention, inaddition to the second feature, a subsidiary connecting rod relativelymovably accommodated in a main connecting rod which connects the mainpiston and the crankshaft to each other is connected at an upper endthereof to the sub-piston, and a swingable member swingable in operativeassociation with the rotation of the crankshaft is pivotally supportednear a lower end of the main connecting rod, the subsidiary connectingrod being connected at a lower end thereof to the swingable member.

With the sixth feature of the present invention, the sub-pistonsupported on the main piston movable within the cylinder bore can bemoved up and down in operative association with the crankshaft.

According to a seventh aspect and feature of the present invention, inaddition to the sixth feature, the swingable member is in cam engagementwith the crankshaft for swinging movement.

With the seventh feature of the present invention, the swingable membercan be swung in precise operative association with the rotation of thecrankshaft.

According to an eighth aspect and feature of the present invention, inaddition to the seventh feature, the swingable member is swung one timefor every one rotation of the crankshaft, and the volume of the air-fuelmixture cooling chamber is increased and decreased one time for everyone rotation of the crankshaft.

With the eighth feature of the present invention, the volume of theair-fuel mixture cooling chamber can be controlled to the timing ofignition of the 2-cycle engine.

According to a ninth aspect and feature of the present invention, inaddition to the first feature, a spark plug is disposed at asubstantially central portion of the combustion chamber, and thecombustion chamber communicates at a peripheral edge thereof with theair-fuel mixture cooling chamber.

With the ninth feature of the present invention, the unburned air-fuelmixture which is in a high-temperature state and which is resident atthe peripheral edge of the combustion chamber can be efficientlyintroduced under a pressure into the air-fuel mixture cooling chamber.

According to a tenth aspect and feature of the present invention, inaddition to the ninth feature, the air-fuel mixture cooling chamber isdefined at a substantially constant gap between an upper surface of amain piston and a lower surface of a sub-piston.

With the tenth feature of the present invention, the surface area of theair-fuel mixture cooling chamber can be ensured to the maximum relativeto a volume thereof to enhance the effect of cooling of the air-fuelmixture.

According to an eleventh aspect and feature of the present invention, inaddition to the first feature, a partition plate is supported movably inupward and downward direction in the cylinder head to face an uppersurface of a piston; the combustion chamber is defined between the uppersurface of the piston and a lower surface of the partition plate, andthe air-fuel mixture cooling chamber is defined at an upper surface ofthe partition plate.

With the eleventh feature of the present invention, the structure of adriving mechanism of the partition plate can be simplified as comparedwith a case where the partition plate is mounted on a movable membersuch as a piston.

According to a twelfth aspect and feature of the present invention, inaddition to the eleventh feature, a spark plug is disposed at one ofdiametrically opposite ends of the combustion chamber, and thecombustion chamber communicates at the other of the diametricallyopposite ends with the air-fuel mixture cooling chamber.

With the twelfth feature of the present invention, the unburned air-fuelmixture which is in a high-temperature state and which is resident atthe other of the diametrically opposite ends of the combustion chambercan be efficiently introduced under a pressure into the air-fuel mixturecooling chamber.

The above and other objects, features and advantages of the inventionwill become apparent from the following description of the preferredembodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 9 illustrate a first embodiment of the present invention,wherein

FIG. 1 is a vertical sectional view of an essential portion of anengine;

FIG. 2 is a sectional view taken along a line 2--2 in FIG. 1;

FIG. 3 is a sectional view taken along a line 3--3 in FIG. 2;

FIG. 4 is an enlarged view of an essential portion shown in FIG. 2;

FIG. 5 is an enlarged view of an essential portion shown in FIG. 2;

FIG. 6 is a graph illustrating the relationship between the crank angleand the height of an air-fuel mixture cooling chamber;

FIGS. 7A to 7C are illustrations for explaining the operation;

FIG. 8 is a graph illustrating the heat generation rate with respect tothe crank angle;

FIG. 9 is a P-V diagram;

FIGS. 10 to 12 illustrate a second embodiment of the present invention,wherein

FIG. 10 is a vertical sectional view of an essential portion of anengine;

FIG. 11 is a sectional view taken along a line 11--11 in FIG. 10;

FIG. 12 is a graph illustrating the relationship between the crank angleand the height of an air-fuel mixture cooling chamber, wherein in thesecond embodiment, members or portions corresponding to those in thefirst embodiment are designated by the same reference characters as inthe first embodiment;

FIGS. 13 and 14 illustrate a third embodiment of the present invention,wherein

FIG. 13 is a vertical sectional view of an essential portion of anengine;

FIG. 14 is a sectional view taken along a line 14--14 in FIG. 13,wherein even in the third embodiment, members or portions correspondingto those in the first and second embodiment are designated by the samereference characters as in the first and second embodiments;

FIG. 15 is a graph illustrating the theoretic thermal efficiency in anOtto-cycle engine.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the present invention will now be described withreference to FIGS. 1 to 12.

As shown in FIGS. 1 to 3, a 4-cycle and single-cylinder engine Eincludes a crankcase 1, a cylinder block 2 and a cylinder head 3. A mainpiston 4 is slidably received in a cylinder bore 2₁ defined in thecylinder block 2 and is pivotally supported on a smaller end of a mainconnecting rod 5 through bisected piston pins 6, 6. A crankshaft 7 isbisected into a first crankshaft half 8 and a second crankshaft half 9.Journal portions 8₁ and 9₁ of the crankshaft halves 8 and 9 arerotatably supported through bearings between bearing support portions1₁, 1₁ of the crankcase 1 and bearing caps 11', 11' fixed to the bearingsupport portions 1₁, 1₁ by bolts 10. The main connecting rod 5 ispivotally supported at its larger end on a pin portion 8₂ provided onthe first crankshaft half 8 of the crankshaft 7 and held in place bybearing cap 11 and bolts 10.

An intake port 13 and an exhaust port 14 are defined in the cylinderhead 3 and open into a combustion chamber 12, and an intake valve bore13₁ and an exhaust valve bore 14₁ are opened and closed by an intakevalve 15 and an exhaust valve 16, respectively. A spark plug 17 ismounted in the cylinder head 3 to face a central portion of thecombustion chamber 12.

As can be seen from reference to FIGS. 1 to 3 in combination with FIG.4, a leg 19₁ of a sub-piston 19 is supported slidably in upward anddownward direction in a cylindrical bushing 18 mounted along a centeraxis of the main piston 4. The leg 19₁ is integrally provided at itsupper end with an umbrella-like portion 19₂ having substantially thesame diameter as the main piston 4, so that a lower surface of theumbrella-like portion 19₂ can be moved toward and away from an uppersurface of the main piston 4. When the sub-piston 19 is moved upwardsrelative to the main piston 4, a conical air-fuel mixture coolingchamber 20 is defined between the sub-piston 19 and the main piston 4 tospread obliquely and upwards to communicate with a peripheral edge ofthe combustion chamber 12.

The structure of a cooling-chamber volume changing means 31 forincreasing and decreasing the volume of the air-fuel mixture coolingchamber 20 will be described with reference to FIGS. 4 and 5.

As shown in FIG. 4, the main connecting rod 5 is hollow, and asubsidiary connecting rod 21 is loosely fitted in the main connectingrod 5 and pivotally supported at its upper end on the leg 19₁ of thesub-piston 19 through a pin 22.

As shown in FIG. 5, a ring-like cam member 23 is rotatably carried on anouter periphery of the pin portion 8₂ of the crankshaft 7. Defined onopposite sides of the cam member 23 sandwiching the main connecting rod5 are a pair of outer peripheral cam surfaces 23₁, 23₁ positioned atradially outer locations, and a pair of inner peripheral cam surfaces23₂, 23₂ positioned at radially inner locations. A pair of rollers 25,25 as cam followers are supported at opposite ends of a pin 24 securedto a lower end of the subsidiary connecting rod 21 of the sub-piston 19,so that they are rolled along the pair of outer peripheral cam surfaces23₁, 23₁. A pair of slippers 26, 26 as cam followers are also supportedon the pin 24, so that they are slid along the pair of inner peripheralcam surfaces 23₂, 23₂.

Each of the slippers 26 is moved up and down while being guided on apair of guide portions 5₁, 5₁ (see FIG. 3) projectingly provided onsides of the main connecting rod 5. A bushing 27 is mounted at slidingcontact portions of the cam member 23 and the larger end of the mainconnecting rod 5. In FIG. 5, a reference character 29 is a key whichconnects the first crankshaft half 8 and the second crankshaft half 9 toeach other, and a reference character 30 is a key which integrallyconnects the cam member 23 that is divided into two members.

As can be seen from FIGS. 1 and 2, a first gear G₁ is fixed to thecrankcase 1 to surround the outer periphery of the journal portion 9₁ ofthe second crankshaft half 9 of the crankshaft 7. A second gear G₂ and athird gear G₃ integrally coupled to each other by a bolt 28 arerotatably carried on a balance weight portion 9₂ of the secondcrankshaft half 9. The second gear G₂ is meshed with the first gear G₁,and the third gear G₃ is meshed with a fourth gear G₄ which isintegrally formed on the cam member 23.

The numbers Z₁, Z₂, Z₃ and Z₄ of teeth of the first, second, third andfourth gears G₁, G₂, G₃ and G₄ are set as follows: Z₁ =45, Z₂ =36, Z₃=28 and Z₄ =70. Therefore, when the crankshaft 7 is rotated in adirection of an arrow A in FIG. 1, the second and third gears G₂ and G₃are rotated in a direction of an arrow B, and the fourth gear G₄ isrotated in a direction of an arrow C. In this case, the cam member 23performs one rotation for every two rotations of the crankshaft 7,because (Z₂ /Z₁)×(Z₄ /Z₃)=2.

When the cam member 23 is rotated, the subsidiary connecting rod 21 withthe rollers 25, 25 guided on the outer peripheral cam surfaces 23₁, 23₁and with the slippers 26, 26 guided on the inner peripheral cam surfaces23₂, 23₂ is moved up and down relative to the main connecting rod 5.This causes the umbrella-like portion 19₂ of the sub-piston 19 with theleg 19₁ guided on the bushing 18 to be moved to or away from the mainpiston 4, whereby the height of the air-fuel mixture cooling chamber 20defined between the lower surface of the umbrella-like portion 19₂ ofthe sub-piston 19 and the upper surface of the main piston 4 is varied,for example, in a range of 0 mm to 0.7 mm.

As shown in the graph of FIG. 6, if the crank angle θ at a bottom deadcenter at which a compression stroke is started is defined as θ=0°, theair-fuel mixture cooling chamber 20 is open during the compressionstroke at a crank angle θ equal to 0° to 180°, and started to be closedfrom near a top dead center immediately after ignition and completelyclosed near a crank angle θ=220° in an expansion stroke. In a secondhalf of the expansion stroke and in an exhaust stroke, the air-fuelmixture cooling chamber 20 remains closed, and in an intake stroke, theair-fuel mixture cooling chamber 20 is opened again at a crank angle θin a range of 540° to 620°.

The operation of the embodiment of the present invention having theabove-described arrangement will be described below.

When the crankshaft 7 is rotated in the direction of the arrow A in FIG.1, the cam member 23 performs one rotation for every two rotations ofthe crankshaft 7 in the direction of the arrow C, as described above. Asis apparent from the graph in FIG. 6, in the compression stroke at acrank angle θ=0° to 180°, the subsidiary connecting rod 21 with therollers 25, 25 guided on the outer peripheral cam surfaces 23₁, 23₁ ofthe cam member 23 and with the slippers 26, 26 guided on the innerperipheral cam surfaces 23₂, 23₂ is pushed upwards relative to the mainconnecting rod 5. As a result, the sub-piston 19 connected to the upperend of the subsidiary connecting rod 21 is relatively lifted relative tothe main piston 4, thereby defining the air-fuel mixture cooling chamber20 having a height of, for example, 0.7 mm between the upper surface ofthe main piston 4 and the lower surface of the umbrella-like portion 19₂of the sub-piston 19.

From near the top dead center (θ=180°) at which the expansion stroke isstarted, the subsidiary connecting rod 21 with the rollers 25, 25 andslippers 26, 26 guided on the cam member 23 is pulled down relative tothe main connecting rod 5, thereby causing the height of the air-fuelmixture cooling chamber 20 to be gradually decreased. At near θ=220°,the upper surface of the main piston 4 abuts against the lower surfaceof the umbrella-like portion 19₂ of the sub-piston 19, thereby causingthe height of the air-fuel mixture cooling chamber 20 to become 0 mm.Thereafter, the height of the air-fuel mixture cooling chamber 20 ismaintained at 0 mm for a period from the second half of the expansionstroke through the exhaust stroke to the start of the intake stroke(220°<θ<540°). Then, at a first half of the intake stroke (540°<θ<600°),the height of the air-fuel mixture cooling chamber 20 is graduallyincreased from 0 mm to 0.7 mm and maintained at 0.7 mm until thecompression stroke is completed.

A portion of an air-fuel mixture drawn through the intake port 13 intothe combustion chamber 12 in the intake stroke is also charged into theair-fuel mixture cooling chamber 20 having an increased volume. Theair-fuel mixture cooling chamber 20 has an extremely large surface areasubstantially equal to a sum of a surface area of the upper surface ofthe main piston 4 and a surface area of the lower surface of thesub-piston 19, irrespective of an extremely small volume thereof.Therefore, when the air-fuel mixture is adiabatically compressed in thecompression stroke, a heat transfer from the air-fuel mixture in theair-fuel mixture cooling chamber 20 to the sub-piston 19 and the mainpiston 4 is promoted to suppress the excessive rising of the temperatureof the air-fuel mixture.

As shown in FIG. 7A, the spark plug 17 mounted at the central portion ofthe combustion chamber 12 generates a spark near the top dead center, asurface of flame of the air-fuel mixture fired at the central portion ofthe combustion chamber 12 is propagated toward the peripheral edge ofthe combustion chamber 12. At this time, the air-fuel mixture in theair-fuel mixture cooling chamber 20 is maintained cool duringcompression and initial firing by the heat transfer to the sub-piston 19and the main piston 4 and hence, cannot be self-fired without waitingfor the arrival of the flame surface, thereby previously inhibiting thegeneration of a knocking.

The self-firing of the air-fuel mixture existing at locations spacedapart from the spark plug 17 (i.e., within the air-fuel mixture coolingchamber 20 and on the peripheral edge of the combustion chamber 12) isprevented as described above and therefore, if the compression ratio isset at a higher value, or even if the ignition timing is not speciallyretarded, the generation of the knocking can be avoided, whereby theenhancement in the thermal efficiency and the prevention of thegeneration of the knocking can be effectively compatible.

As shown in FIG. 7B, while the surface of flame of the fired air-fuelmixture is propagated from the central portion of the combustion chamber12 toward the peripheral edge, the air-fuel mixture in the air-fuelmixture cooling chamber 20 is pushed to the peripheral edge of thecombustion chamber 12 by reducing the volume of the air-fuel mixturecooling chamber 20, and is burned thereat. In this way, the burning ofthe air-fuel mixture is not performed in the air-fuel mixture coolingchamber 20, but is performed only in the combustion chamber 12 havingthe surface area which is small as compared with its volume. Therefore,it is possible to suppress the loss in cooling of the combustion gas tothe minimum to enhance the thermal efficiency. In addition, it ispossible to control the duration of the burning to the completion of theburning of all the air-fuel mixture and a rate of heat generated fromthe air-fuel mixture with the lapse of the time to any extent, as shownin FIG. 8, by changing the setting of the amount of air-fuel mixtureinjected from the air-fuel mixture cooling chamber 20 into thecombustion chamber 12 and the timing of the injection.

Thus, it is possible to slowly conduct the burning to drop the maximumtemperature within the combustion chamber 12, thereby reducing theamount of NO_(x) in an exhaust gas and reducing the variation inpressure in the combustion chamber 12 to prevent the generation of avibration and a noise.

As shown in FIG. 7C, the air-fuel mixture cooling chamber 20 iscompletely closed in the exhaust stroke and hence, the combustion gascannot be retained in the air-fuel mixture cooling chamber 20, and whenthe air-fuel mixture cooling chamber 20 is opened in the next intakestroke, only the air-fuel mixture can be filled thereinto.

In the P-V diagram in FIG. 9, a dot-dash line corresponds to the priorart engine wherein the generation of a knocking is prevented bydecreasing the compression ratio and retarding the ignition timing. Ifonly the compression ratio is increased without application of thepresent invention to the prior art engine, the maximum pressure is risento P₁, as shown by a broken line, thereby permitting the generation of aknocking. However, according to the present invention shown by a solidline, the burning of the air-fuel mixture drawn and charged into theair-fuel mixture cooling chamber 20 is retarded and moreover, thecombustion chamber 20 functions as a pressure buffering chamber.Therefore, the maximum pressure due to the burning is increased only toP₂, thereby preventing the generation of a knocking reliably bycooperation with the effect of cooling the air-fuel mixture by theair-fuel mixture cooling chamber 20. Thereafter, the pressure ismaintained at a higher level without being increased in the form near tothe constant-pressure burning in a diesel engine, by continuation of theburning of the air-fuel mixture discharged from the air-fuel mixturecooling chamber 20.

The cycle of the engine according to the embodiments of the presentinvention departs from a theoretic Otto cycle due to a reduction ineffective pressure with the above-described slow burning. However, it ispossible to increase the compression ratio and to set the ignitiontiming at near an optimal ignition timing by the prevention of theknocking, thereby remarkably enhancing the thermal efficiency, and tomake up for disadvantages due to the departing from the Ofto cycle toprovide advantages.

A second embodiment of the present invention will be described belowwith reference to FIGS. 10 to 12.

As shown in FIGS. 10 and 11, the engine E is a 2-cycle andsingle-cylinder engine of a case reed valve type. The engine E includesa reed valve 35 mounted in a wall surface of a crankcase 1 to permit thecommunication of the inside and outside of the crankcase 1 with eachother, and a pair of scavenging passages 2₃, 2₃ which are provided in awall surface of a cylinder block 2 to connect a pair of scavenging ports2₂, 2₂ opening into a cylinder bore 2₁ to a crank chamber.

As in the first embodiment, the second embodiment includes a sub-piston19 which is supported movably in upward and downward direction on a mainpiston 4 through a leg 19₁. An air-fuel mixture cooling chamber 20communicating with a peripheral edge of the combustion chamber 12 isdefined between an upper surface of the main piston 4 and a lowersurface of an umbrella-like portion 19₂ of the sub-piston 19.

The structure of a cooling-chamber volume changing means 31 which isoperable to increase and decrease the volume of the air-fuel mixturecooling chamber 20 will be described below. A subsidiary connecting rod21 is relatively movably accommodated in a main connecting rod 5 andconnected at its upper end to the leg 19₁ of the sub-piston 19 through apin 22. A bifurcated fork-like swingable member 37 is pivotallysupported at its upper portion near the larger end of the mainconnecting rod 5 through a pin 36, and the subsidiary connecting rod 21is pivotally supported at its lower end on the swingable member 37through a pin 38.

A cam groove 7₂ is provided in an outer periphery of a pin portion 7₁ ofthe crankshaft 7, and a pair of cam followers 39, 39 mounted at a lowerend of the swingable member 37 abut against the cam groove 7₂.

According to the second embodiment, whenever the crankshaft 7 performsone rotation, the swingable member 37 is swung in one reciprocationabout the pin 36 through the cam groove 7₂ and the cam followers 39, 39,and the sub-piston 19 connected to the swingable member 37 through thesubsidiary connecting rod 21 is moved up and down in one reciprocationrelative to the main piston 4, whereby the volume of the air-fuelmixture cooling chamber 20 is varied with the rotation of the crankshaft7.

As is apparent from FIG. 12, if the crank angle θ at the bottom deadcenter is defined as being equal to 0°, the volume of the air-fuelmixture cooling chamber 20 is set, so that it is started to be increasednear a crank angle θ equal to 90°; becomes the maximum value near acrank angle θ equal to 140° slightly short of the top dead center and isthen gradually decreased till near a crank angle θ equal to 190°slightly passing the top dead center. In the 2-cycle engine in which oneexplosion is conducted for every one rotation of the crankshaft 7, theincreasing and decreasing of the volume of the air-fuel mixture coolingchamber 20 are carried out near each top dead center in the abovemanner.

As a result, as in the previously-described first embodiment, even ifthe compression ratio is increased to enhance the thermal efficiency,the generation of the knocking can be prevented by maintaining theair-fuel mixture cool that is drawn and charged into the air-fuelmixture cooling chamber 20 by the heat transfer from the wall surface ofthe air-fuel mixture cooling chamber 20 during the compression of theair-fuel mixture within the combustion chamber 12. By discharging theair-fuel mixture from the air-fuel mixture cooling chamber 20 decreasedin volume to burn the air-fuel mixture during propagation of the surfaceof flame of the air-fuel mixture fired by the spark plug 17, the burningof the air-fuel mixture can be conducted more slowly over a long timethan the burning in the prior art, which can contribute to a reductionin the amount of NO_(x) and reductions in noise and vibration.

A third embodiment of the present invention will be described below withreference to FIGS. 13 and 14.

In the third embodiment, the present invention is applied to a 4-cycleengine of a side valve type. The engine E includes a combustion chamber12 which has a portion extending laterally in one radial direction froma cylinder bore 2₁, and the inlet valve 15, the exhaust valve 16 and thespark plug 15 are installed in that extending portion of the combustionchamber. A partition plate 41 is disposed movably in upward and downwarddirection in the cylinder head 3 to face the combustion chamber 12. Thecombustion chamber 12 is defined between an upper surface of a piston 42and a lower surface of the partition plate 41, and an air-fuel mixturecooling chamber 20 having a small height is defined above an uppersurface of the partition plate 41. An arcuate notch 41₁ is formed at anend of the partition plate 41 farthest from the spark plug 17 to permitthe communication between the combustion chamber 12 and the air-fuelmixture cooling chamber 20 (see FIG. 14). The partition plate 41 issupported movably in upward and downward direction in the cylinder head3 through a guide rod 43 and biased upwards by a return spring 44.

A cam shaft 45 is mounted above the partition plate 41 and connected tothe crankshaft through a transmitting mechanism (not shown), so that itis rotated at a number of rotations which is one half of a number ofrotations of the crankshaft. A cam 46 fixed to the cam shaft 45 abutsagainst a cam follower 47 fixed to an upper end of the guide rod 43.While the crankshaft performs two rotations, the volume of the air-fuelmixture cooling chamber 20 is increased and decreased one time, as shownin the graph in FIG. 6.

According to the third embodiment, the same operation and effect asthose in the first embodiment can be provided, but also the structure ofthe cooling-chamber volume changing means 31 for increasing anddecreasing the volume of the air-fuel mixture cooling chamber 20 can besubstantially simplified, as compared with the case when the air-fuelmixture cooling chamber 20 is defined above the upper surface of themain piston 4 as in the first embodiment.

Although the embodiments of the present invention have been described indetail, it will be understood that the present invention is not limitedto the above-described embodiments, and various modifications in designmay be made without departing from the spirit and scope of the inventiondefined in claims.

For example, the timing of increasing and decreasing the volume of theair-fuel mixture cooling chamber 20 and the amount of volume changed arenot limited to those in the embodiments, and can be changed to anyextent depending upon the characteristic of the engine.

What is claimed is:
 1. A combustion control system for an engine,comprising an air-fuel mixture cooling chamber which communicates with acombustion chamber defined in a cylinder head, so that an air-fuelmixture can flow from the air-fuel mixture cooling chamber into thecombustion chamber and vice versa, and a cooling-chamber volume changingmeans for increasing and decreasing a volume of the air-fuel mixturecooling chamber in operative association with a rotation of acrankshaft, wherein the cooling-chamber volume changing means isoperable to decrease the volume of the air-fuel mixture cooling chamberduring burning of the air-fuel mixture in the combustion chamber.
 2. Acombustion control system for an engine according to claim 1, furtherincluding a sub-piston supported movably in upward and downwarddirection on a main piston which is slidably received in a cylinderbore, said air-fuel mixture cooling chamber being defined between anupper surface of said main piston and a lower surface of saidsub-piston.
 3. A combustion control system for an engine according toclaim 2, further including a subsidiary connecting rod relativelymovably accommodated in a main connecting rod which connects said mainpiston and said crankshaft to each other, said subsidiary connecting rodbeing connected at an upper end thereof to said sub-piston and at alower end thereof to a cam member which is rotatably supported on saidcrankshaft.
 4. A combustion control system for an engine according toclaim 3, wherein said cam member is connected to a casing through a geartrain, so that the cam member is rotated in operative association withthe rotation of said crankshaft.
 5. A combustion control system for anengine according to claim 4, wherein said cam member performs onerotation for every two rotations of the crankshaft, and the volume ofthe air-fuel mixture cooling chamber is increased and decreased one timefor every two rotations of the crankshaft.
 6. A combustion controlsystem for an engine according to claim 2, further including asubsidiary connecting rod which is relatively movably accommodated in amain connecting rod connecting the main piston and the crankshaft toeach other and which is connected at an upper end thereof to thesub-piston, and a swingable member which is swingable in operativeassociation with the rotation of the crankshaft and which is pivotallysupported near a lower end of the main connecting rod, said subsidiaryconnecting rod being connected at a lower end thereof to the swingablemember.
 7. A combustion control system for an engine according to claim6, wherein said swingable member is in cam engagement with saidcrankshaft for swinging movement.
 8. A combustion control system for anengine according to claim 7, wherein said swingable member is swung onetime for every one rotation of said crankshaft, and the volume of saidair-fuel mixture cooling chamber is increased and decreased one time forevery one rotation of said crankshaft.
 9. A combustion control systemfor an engine according to claim 1, further including a spark plug whichis disposed at a substantially central portion of the combustionchamber, said combustion chamber communicating at a peripheral edgethereof with said air-fuel mixture cooling chamber.
 10. A combustioncontrol system for an engine according to claim 9, wherein said air-fuelmixture cooling chamber is defined at a substantially constant gapbetween an upper surface of the main piston and a lower surface of asub-piston.
 11. A combustion control system for an engine according toclaim 1, further including a partition plate which is supported movablyin upward and downward direction in the cylinder head to face an uppersurface of a piston, said combustion chamber being defined between theupper surface of the piston and a lower surface of said partition plate,said air-fuel mixture cooling chamber being defined above an uppersurface of said partition plate.
 12. A combustion control system for anengine according to claim 11, further including a spark plug which isdisposed at one of diametrically opposite ends of the combustionchamber, said combustion chamber communicating at the other of thediametrically opposite ends with said air-fuel mixture cooling chamber.13. A combustion control system for an internal combustion engine havinga combustion chamber defined in a cylinder of the engine between a mainpiston and a cylinder head, comprising means for forming an air-fuelmixture cooling chamber between the main piston and the cylinder headseparate from a main portion of the combustion chamber, and means forincreasing and decreasing a volume of the air-fuel mixture coolingchamber in operative association with movement of the piston fordecreasing the volume of the air-fuel mixture cooling chamber duringburning of the air-fuel mixture in the combustion chamber.
 14. Acombustion control system for an engine according to claim 13, whereinsaid means for forming an air-fuel mixture cooling chamber includes asub-piston supported on a top of the main piston for movement toward andaway from the main piston, said air-fuel mixture cooling chamber beingdefined between an upper surface of the main piston and a lower surfaceof said sub-piston.
 15. A combustion control system for an engineaccording to claim 14, further including a subsidiary connecting rodmovable relative to the main piston, said subsidiary connecting rodbeing connected at an upper end thereof to said sub-piston and at alower end thereof to a crankshaft through means for causing saidrelative movement.
 16. A combustion control system for an engineaccording to claim 14, wherein said sub-piston is of a smaller diameterthan the main piston for forming a circumferential inlet and outlet awall of the cylinder for the air-fuel mixture cooling chamber.
 17. Acombustion control system for an engine according to claim 13, furtherincluding a spark plug which is disposed at a substantially centralportion of the combustion chamber, said combustion chamber communicatingat a peripheral edge thereof with said air-fuel mixture cooling chamber.18. A combustion control system for an engine according to claim 17,wherein said air-fuel mixture cooling chamber is defined at asubstantially constant gap between an upper surface of the main pistonand a lower surface of a sub-piston.
 19. A combustion control system foran engine according to claim 13, further including a partition platewhich is supported movably in an upward and downward direction in thecylinder head to face an upper surface of the main piston, saidcombustion chamber being defined between the upper surface of the pistonand a lower surface of said partition plate, said air-fuel mixturecooling chamber being defined above an upper surface of said partitionplate.
 20. A combustion control system for an engine according to claim19, further including a spark plug which is disposed at one ofdiametrically opposite sides of the combustion chamber, said combustionchamber communicating at the other of the diametrically opposite sideswith said air-fuel mixture cooling chamber.